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The apparent rediscovery of the Ivory-billed Woodpecker Campephilus principalis in Arkansas, USA, …


Biology Articles » Zoology » Ornithology » Video analysis of the escape flight of Pileated Woodpecker Dryocopus pileatus: does the Ivory-billed Woodpecker Campephilus principalis persist in continental North America? » Discussion

Discussion
- Video analysis of the escape flight of Pileated Woodpecker Dryocopus pileatus: does the Ivory-billed Woodpecker Campephilus principalis persist in continental North America?

Evidence is presented here to show that the distinctive plumage features of Pileated Woodpecker are surprisingly difficult to resolve in poor-quality video of birds in escape flight away from the camera, and that they can show apparent plumage patterns that might more readily be associated with Ivory-billed Woodpecker. Irrespective of the identity of the bird in the Luneau video, this knowledge will be critical to assessment of further claims of Ivory-billed Woodpeckers during the current intensive search effort. It is, however, suggested here that critical frames used for identification of the Luneau video woodpecker as an Ivory-billed Woodpecker are also consistent with Pileated Woodpecker. The wingbeat frequency of the bird in the Luneau video is also perhaps consistent with Pileated Woodpecker, at least for short periods of flight.

Analysis of the videos of Pileated Woodpecker has supported the hypothesised interpretations of key frames of the Luneau video by Sibley et al [4]. Although the rebuttal of that comment in Fitzpatrick et al [5] asserted that flexion and motion of wings of Pileated Woodpeckers could not produce the images seen in the Luneau video, it has been shown here that they can.

The Luneau video as presented in Fitzpatrick et al [1], shows features that are consistent with Pileated Woodpecker, and inconsistent with Ivory-billed Woodpecker. It is argued in this paper that, in fact, the black trailing edge of the wing of a Pileated Woodpecker is seen clearly in the Luneau video, during the downstroke of the wingbeat cycle, but that it has been misinterpreted as black wingtips (Figure 1, 2, 3).

A fuller analysis of the Luneau video by the Cornell University team is presented online [7]. Although it is not peer-reviewed, the points this article makes should be taken into account. The authors summarise nine diagnostic traits from their analysis of the Luneau video that identify the bird as Ivory-billed Woodpecker. These are listed and discussed point-by point below.

1. 'The underwing pattern in flight consistently appears largely white, giving the appearance of having black wingtips but lacking any black along the rear, or trailing edge.'

Data presented in this paper show that this statement is not wholly supported, and in any case the underwing of Pileated Woodpeckers can present the same appearance.

2. 'The upperwing pattern in flight consistently shows a broad, white trailing edge, with no frames demonstrating the conspicuous dark rear border to be expected of normal Pileated Woodpeckers.'

Notwithstanding that certain frames of the Luneau video (e.g. frame 350) do appear to show a black trailing edge to the upperwing, data presented in this paper shows that, at this angle of view and resolution of video, Pileated Woodpeckers also may fail to show this feature. This analysis has shown that the hypothesis presented in Sibley et al [4] is plausible, i.e. that some of the frames interpreted by [1] to show the upperwing of an Ivory-billed Woodpecker may in fact show large amounts of white and the black trailing edge from the underwing of a Pileated Woodpecker.

3. 'The wings are longer relative to the body diameter than in Pileated Woodpecker and consistent with the wing shape of Ivory-billed Woodpecker.'

Fitzpatrick et al [5] agreed that accurate measurements were not possible from the video images presented in their original paper [1], and it seems unlikely that much confidence can be placed in the wing-length measurements of the bird in the Luneau video. Comparison of, for example, Figure 1A, frame 283.3 with Figure 1B, frame 578 suggests that any differences will be very difficult to prove.

4. 'Reenactment of the scene using life-sized, realistically painted, dynamically flapping models produced images remarkably similar to those of the Luneau video using the Ivory-billed Woodpecker model, and images clearly identifiable as Pileated Woodpecker using a model of that species.'

Interpretation of model re-enactments is hampered by the fact that the stiff, flat-winged models cannot reflect the wing flexion and curvature of real birds. Reenactment of the scene using real Pileated Woodpeckers has produced images remarkably similar to the Luneau video.

5. 'The wingbeat frequency is 8.6 beats per second, which is almost identical to that recorded for Ivory-billed Woodpecker (as documented by one acoustic record from 1935). The wing-beat frequencies of Pileated Woodpecker are not known to exceed 7.5 beats per second, and more typically range between 3 and 6 beats per second.'

The fact that in only four recorded escape flights of Pileated Woodpecker, two were recorded for which the initial escape flight wingbeat frequency (8.0 s-1 and 8.6 s-1) exceeded that previously recorded for this species shows that previous datasets were too limited to make this conclusion. Birds flap more rapidly at take off to gain altitude and speed than they do in sustained level flight: Pileated Woodpecker flight data in the literature [1,4,5] was derived from the work of Tobalske [8], which explicitly excluded the initial take-off period, and therefore cannot be used to support the elimination of Pileated Woodpecker in the Luneau video. Furthermore, the bird in the Luneau video is consistently gaining height from a low position above water and, whatever its species, might be expected to flap more rapidly than if it were in level flight. Tanner [9] noted that Pileated Woodpeckers can maintain extended fast direct flight. He was of the opinion that flight pattern was not a useful character for separating the two species in the field, and that Pileated Woodpeckers frequently fly in a manner that was in no way different to Ivory-billed Woodpeckers.

The figure of 8.6 wingbeats per second for the Luneau bird (data reanalysed here) is taken as consistent with Ivory-billed Woodpecker on the basis of analysis of a single archival audio recording [3]. The Ivory-billed Woodpecker in that audio tape is clearly flapping its wings, but without accompanying visual confirmation it is not clear that it is in flight. In general, larger birds are expected to flap their wings more slowly than smaller birds of comparable wing morphology. Tobalske [8] showed that, across species, smaller woodpeckers tend to flap more quickly than larger ones, and that there was considerable intraspecific variation. The assertion that Ivory-billed Woodpeckers flap their wings more quickly than Pileated Woodpeckers is therefore counterintuitive. Further comment is conjecture: while the flight pattern and wing posture of the bird in the Luneau video may be unusual, it has not been shown that it is outside the range of variability of Pileated Woodpecker, and cannot therefore be used to eliminate the possibility that it was the commoner species.

6. 'White plumage on the back is visible on the retreating bird as it begins to gain altitude. Ivory-billed Woodpecker has white on the back; Pileated Woodpecker has entirely black back.'

This was discussed by Sibley et al [4], who argued that the images thought to show white on the dorsum were too small to be accepted uncritically. In all the frames of the Luneau video that appear to show white on the dorsum, the bird is distant (dorsal white is not visible on the higher resolution images earlier in the video) and partially obscured, making it difficult to distinguish dorsum from wing. Spurious areas of white pixels appear as artifacts in both videos. Nevertheless, this remains the best evidence that the Luneau bird was not a standard Pileated Woodpecker.

7. 'The dorsal view of the right wing as it begins to unfold shows a triangle of white that matches in size and position the white on the folded wing of an Ivory-billed Woodpecker beginning to launch into flight.'

No further comment is provided here. An alternative explanation was offered by Sibley et al [4] and rebutted by Fitzpatrick et al [5]. The statement requires a degree of certainty about the position of the wing.

(8) 'The distance between the wrist area and the tip of the tail (32–36 cm, as measured when the bird begins to take flight) is comparable to known measurements of Ivory-billed Woodpecker and considerably larger than even the largest Pileated Woodpecker we measured.'

As stated under (3), above, there is general agreement that accurate measurements are not possible from the Luneau video because too many uncontrolled variables are involved [4,5].

9. 'Only 20 seconds before the woodpecker flees, a bird with the size and color pattern of an Ivory-billed Woodpecker was perched within 3 m of the site from which the woodpecker took flight.'

This would be a strong argument if it could be shown that the object in question was a bird and not, as is now apparently thought likely, a section of branch or tree stump [10]. The Luneau video reveals several white triangular patches apparently visible on or around tree trunks, most or all of which must therefore be images of tree topography or video artifacts. This was discussed in the literature (see [4,5]).

Central to the identification of the flying bird seen in the Luneau video was the evidence that plumage and flight patterns were inconsistent with Pileated Woodpecker. A very basic video analysis presented here has suggested that this may not be the case, and that further research is needed. Any identification of the bird in the Luneau video as an Ivory-bill must take into account the data presented here and in Sibley et al [4], which shows it is largely consistent with Pileated Woodpecker and points out apparent inconsistencies with Ivory-billed Woodpecker. This does not of course necessarily imply that the Ivory-billed Woodpecker is extinct, nor indeed entirely rule out the possibility that the bird in the Luneau video was one. There appears to be no reason to question the anecdotal sight records of Ivory-billed Woodpecker presented in Fitzpatrick et al [1] (or in many online sources), because some of them appear credible, albeit brief. Audio evidence has since been published [11] although this too is far from conclusive. However, to regard the Luneau video by itself as presenting anything other than an unidentified woodpecker falls below the standards of proof normally required for scientific publication: the images are not good enough. The Ivory-billed Woodpecker may persist in continental North America, and there is enough anecdotal evidence to make this a possibility, but the Luneau video does not support the case. The balance of evidence would suggest that the bird in the Luneau video is more likely to have been a Pileated Woodpecker, but the search for Ivory-billed Woodpecker should continue.

While this paper was under review, a report of sight records and sound recordings of Ivory-billed Woodpeckers was published from a location in Florida [12]. This very exciting claim is strengthened by reports of sighting of the white dorsal stripes on one bird in flight. Unfortunately, several sightings were made without optical aids and cannot be considered proven. The 'kent' calls recorded from the Florida location are spectrographically similar to the 'bleat' calls of young White-tailed Deer, as described in Richardson et al [13]. A clear photograph will be required from this location too before the presence of Ivory-billed Woodpeckers can be considered confirmed. It is hoped that this paper will help with assessment of any further low quality photographs or videos.


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